<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body {
            background: url(images/bgi.jpg)center center no-repeat fixed;
        }

        /* .overlay {
            box-shadow: 0 0 0 999px rgba(0,0,0,.8); 
            box-shadow: 0 0 0 50vmax rgba(0,0,0,.8);
        } */
        

        .lightbox {
            width: 500px;
            height: 200px;
            background: salmon;
            position: absolute;
            z-index: 1;
            top: 30%;
            right: 50%;
        }
    </style>
</head>

<body>
    <div class="overlay">
        <div class="lightbox"></div>
    </div>
</body>

</html>